Kit Must

approximately how much will instillation cost to install 4.1 gears and the bearing kit on a 2001 must gt?

the gears run around 160 and the kit will be another 75

Getting 4.10's in my mustang was around $750, installation was not that much maybe around $150. The rest was spent on a whole new differential which included the 'pumpkin', and gears + trak-loc. All was brand new and an easy installation from what I hear, but your mustang is probably a different setup. Mine is a '68 with an 8 inch and I believe yours is an 8.8 inch. Gas mileage should not be too bad because you have an overdrive tranny, when I installed an overdrive transmission (AOD) in my mustang I averaged 20 mpg on the highway with rpm's around 2500 (for a 302). So you would be looking at around 400-450 dollars total for everything including parts, the 4.10's are well worth it!
hope this helps